1. Field of the Invention
The present invention relates to a gate-insulating film of an MIS-type field-effect transistor or the like and its forming method, particularly to a gate-insulating film having a polycrystalline film made of a metal oxide and its forming method.
2. Description of the Prior Art
In development of a semiconductor device, it is one of the most important problems how to form a gate-insulating film of an MIS-type field-effect transistor at a high reliability and a high controllability. In case of a recent logic-based MIS(metal-insulating-film-semiconductor)-type device, the thickness of a gate-insulating film of a transistor rapidly decreases and an oxide film having a thickness of 3.0 nm or less is used. A silicon oxide film has been used up to now because of a relatively simple process as compared with other processes that the film can be formed by heat-treating a silicon substrate in an oxygen atmosphere in addition to its preferable insulating characteristic and interfacial characteristic. However, when the thickness of the film decreases from 5.0 nm to 4.0 nm or less, a phenomenon having been latent so far is actualized and becomes an obstacle for obtaining a device characteristic same as ever. For example, the phenomenon appears when leak current of a gate-insulating film increases. In case of a silicon oxide film having a low relative permittivity, it is impossible to control tunneling electrons at the time of reducing the thickness of the film.
To reduce the leak current, a method is known which uses a material having a higher relative permittivity as a gate-insulating film instead of a silicon oxide film. For example, Japanese Patent Laid-Open No. 7-231088 discloses an art of using a composite laminate film comprising one of materials having a high permittivity such as Ta2O5, (Ba1xe2x88x92xSrx)TiO3, and PbZr1xe2x88x92xTiO3 and a silicon oxide film having a thickness of 10 nm or less as an insulating film of a MIS-type field-effect transistor.
At the time of using one of these films having a high permittivity as a gate-insulating film, it is possible to increase the physical thickness of the gate-insulating film by a value equivalent to the high permittivity when equalizing the thickness of a silicon oxide film with the electrical film thickness of a dielectric, that is a film thickness converted into silicon-oxide-film thickness. Thus, an effect is obtained that the tunnel distance of an electron increases and a direct tunnel current, that is, a gate leak current does not easily flow.
However, to use a material having a high relative permittivity as a gate-insulating film, many problems must be solved. It is one of the big problems that these high-permittivity insulating films do not have a very large relative permittivity because they are amorphous when deposited and a process for performing a high-temperature treatment in an oxygen atmosphere after deposited is necessary. The influence of a high-temperature treatment on a high-permittivity film in an oxygen atmosphere is, of course, preferable. However, a silicon oxide film is formed on the lower side of a high-permittivity film, that is, on the silicon interface due to the high-temperature heat treatment in an oxygen atmosphere and makes it difficult to decrease the whole electrical thickness (converted into silicon-oxide-film thickness) of a gate-insulating film.
FIG. 7 roughly shows the sequence of a process for forming a high-permittivity gate-insulating film in accordance with a conventional method. In this case, a tantalum oxide film is used as a high-permittivity film. Surface purification (pretreatment 16) for forming a tantalum oxide film on a silicon substrate is performed and then the tantalum oxide film is deposited up to a desired thickness (metal oxide film deposition 26) through CVD. Then, a comparatively-low-temperature heat treatment (low-temperature oxidation treatment 36) is performed in an oxidizing atmosphere in order to introduce oxygen for reinforcement and finally, heat treatment is performed in an oxidizing atmosphere at a high temperature (high-temperature oxidation treatment 46). The final high-temperature oxidation treatment 46 is performed to make the tantalum oxide film polycrystalline and perform reinforcing oxidation. This polycrystallization makes it possible to reduce a gate leak current and improve a relative permittivity.
FIGS. 8A and 8B schematically show sectional views of the tantalum oxide film thus obtained on a silicon substrate 11. FIG. 8A shows a state immediately before the high-temperature oxidation treatment 46, in which a silicon oxide film 12 having a thickness of approx. 0.5 nm formed when an amorphous tantalum oxide film 13 is deposited on the silicon substrate 11, the amorphous tantalum oxide film 13, and an oxygen-rich amorphous tantalum oxide film 14 into which oxygen is additionally introduced through the low-temperature oxidation treatment 36 such as UV/O3 treatment are superposed.
Then, by performing the high-temperature oxidation treatment 46, the tantalum oxide film is crystallized and a polycrystalline tantalum oxide film 15 is formed as shown in FIG. 8B. When a high-permittivity film is crystallized, it easily becomes columnar grains and a crystal is formed which becomes columnar in the film-thickness direction, that is, in which grain boundaries 16 are directly connected each other from the surface up to the silicon substrate.
A problem in this case is that the thickness of the silicon oxide film 12 increases and the thick silicon oxide film 12 is formed on the lower side of the high-permittivity film, that is, on the interface of the silicon substrate. Though crystallization occurs immediately when a high temperature is applied, a certain time is necessary for reinforcement oxidation of the tantalum oxide film. In this case, oxidation species in the atmosphere are diffused by passing through grain boundaries of the crystallized tantalum oxide film to reach the silicon substrate and directly oxidize the silicon substrate. For example, at the time of depositing a tantalum oxide film having a thickness of 8 nm, performing UV/O3 treatment at 500xc2x0 C. for 10 min and moreover performing heat treatment in a dry oxygen atmosphere at 800xc2x0 C., the thickness of a silicon oxide film formed on the interface of the silicon substrate reaches approx. 3.5 nm.
The thickness of a high-permittivity film when used as a gate-insulating film instead of a silicon oxide film is just kept in a range of 3.0 nm or less. Therefore, a requested film thickness cannot be realized due to the 3.5-nm silicon oxide film unavoidably formed.
Of course, it is possible to control the thickness of the silicon oxide film at the interface by lowering the heat-treatment temperature. However, a relative permittivity is not increased as expected but a gate leak current increases. Moreover, grain boundaries directly extending in the film-thickness direction cause the gate leak current to increase.
Objects of the Invention
It is an object of the present invention to provide a gate-insulating film comprising a polycrystalline film made of a metal oxide which has a high relative permittivity, controls a gate leak current to a completely small value, and has a small film thickness converted into silicon-oxide-film thickness (electrical film thickness) of the whole gate-insulating film because formation of a silicon oxide film is controlled at the interface of a silicon substrate and a gate-insulating film forming method.
A gate-insulating film of the present invention is a gate-insulating film having a polycrystalline film made of a metal oxide, wherein a grain boundary plane extending in parallel with a plane of the polycrystalline film is present at the position of a predetermined thickness of the polycrystalline film and grain boundaries extending in the film-thickness direction of polycrystals configuring the polycrystalline film are discontinuous on the grain boundary plane.
Moreover, another gate-insulating film of the present invention is a gate-insulating film having a polycrystalline film made of an oxide, wherein the polycrystalline film has a laminate structure of polycrystalline layers polycrystallized after amorphous layers made of an oxide are independently superposed each other and grain boundaries extending in the thickness direction of polycrystalline layers adjacent each other in the film thickness direction are discontinuous each other.
Still another gate-insulating film of the present invention is a gate-insulating film having a polycrystalline film made of oxide, wherein the polycrystalline film has a structure in which the grain size of the film in the thickness direction of a layer is equal to an average grain size in a direction parallel with the layer, a plurality of polycrystalline layers respectively having a thickness equal to a grain size in the layer-thickness direction are superposed each other, grain boundaries extending in the thickness direction of polycrystalline layers adjacent each other in the layer thickness direction are discontinuous.
A gate-insulating-film forming method of the present invention is a method for forming a gate-insulating film containing a metal oxide, wherein, at the time of defining two consecutive steps such as the step of forming an amorphous layer made of a metal oxide on a semiconductor substrate and the step of oxidizing the amorphous layer at a first temperature in an atmosphere containing oxygen as one cycle, the step of crystallizing the amorphous layer by heat-treating the layer at a second temperature equal to or higher than the first temperature after executing the consecutive steps by at least two cycles is included.
Another gate-insulating-film forming method of the present invention is a method for forming a gate-insulating film made of a metal oxide, wherein the following steps are included: the step of forming a first amorphous layer made of a metal oxide on a semiconductor substrate, the step of forming a second amorphous layer containing oxygen more than the case of the first amorphous layer on the first amorphous layer, the step of forming a third amorphous layer containing oxygen less than the case of the second amorphous layer on the second amorphous layer, and the step of heat-treating the first, second, and third amorphous layers and polycrystallizing them.
According to the above present invention, an amorphous film made of a metal oxide is separated into several layers through the low-temperature oxidation treatment performed while the metal-oxide amorphous film is formed. Each separated amorphous film is simultaneously and independently crystallized through comparatively-high-temperature heat treatment to be performed later at each layer. Therefore, it is possible to decrease the grain size of the crystal of a metal-oxide polycrystalline film.
Decrease of grain size has not only an advantage of controlling the dispersion of characteristics of a high-permittivity film in a wafer plane but also an advantage of controlling a gate leak current. The largest advantage is that it is possible to divide the grain boundary of each layer by dividing grains of a crystalline tantalum-oxide film at each layer and decrease the number of oxidation species reaching a silicon interface at the time of high-temperature oxidation treatment. As a result, it is possible to control the formation of a silicon oxide film (relative permittivity of 3.9) having a relative permittivity lower than a high-permittivity film and form a laminate-type high-permittivity film having a very high relative permittivity as the whole of a gate-insulating film. In other words, it is possible to form a high-permittivity film having a very small thickness converted into silicon-oxide-film thickness.